The present invention relates to an electric double layer capacitor of high electrostatic capacitance which utilizes the principles of an electric double layer that is composed of an interface between activated carbon and electrolyte.
Recently, high-capacitance capacitors based on the electric double layer principles have been developed as backup power supplies for memories in electronic systems, and are widely used with microcomputers and IC memories.
One type of electric double layer capacitor is disclosed in U.S. Pat. No. 3,536,936, for example. FIG. 5 of the accompanying drawings shows the structure of the disclosed electric double layer capacitor. The electric double layer capacitor comprises a single basic cell composed of a pair of current collectors 101 of an electron conductor which serves as a pair of collector electrodes, a pair of carbon electrodes 102 made of activated carbon particles, a pair of nonconductive gaskets 103, and an isolating plate 104 positioned between the carbon electrodes 102 for preventing electrons from moving between the electrodes 102. The carbon electrodes 102 are made as paste electrodes from a concentrated slurry which is a mixture of powdery or particulate activated carbon and an electrolyte. The electrolyte has three functions to perform. It serves as a promotor of ion conductivity, an ion source, and a binder for the carbon particles.
It is important that the internal resistance of an electric double layer capacitor be low. The internal resistance of an electric double layer capacitor is greatly affected by the contact resistance of active carbon of the polarized electrodes and the contact resistance between the collector electrodes and the polarized electrodes.
Therefore, in order to reduce the internal resistance of the polarized electrodes and the contact resistance between the collector and polarized electrodes, each basic cell should be kept under vertical pressure to bring the particles of the paste activated carbon into good electric contact with each other. Conventional electric double layer capacitors require each cell to be kept under a pressure of about 100 kg/cm.sup.2 though it depends on the size of the electrodes, the size of the particles of the carbon material, or the kind of the electrolyte used. In prior electric double layer capacitors, the cells are kept under pressure by deforming the outer cases of the capacitors or bonding the current collectors strongly to gaskets. If an electric double layer capacitor is to be used as a large-capacitance capacitor, e.g., a power supply for energizing a motor, then it is necessary to increase the cross-sectional areas of the electrodes of the basic cell. Therefore, the pressure to be applied to the basic cell has to be increased. Increasing the pressure, however, causes some practical problems such as the selection of means for applying the pressure and the need for high rigidity for the outer cell which houses the basic cell.
In order to solve the above problems, the applicant has proposed an electric double layer capacitor of simple structure which has a high density of stored energy and requires no means for pressurizing the electrodes, as disclosed in Japanese Patent Application No. 1(1989)-215277 (corresponding to U.S. patent application Ser. No. 550,170 and European Patent Application No. 980308781.5). The proposed electric double layer capacitor has polarized electrodes each in the form of a porous sintered body composed of minute active carbon particles which are joined together by sintering.
The density, volume resistance (specific resistance), and specific surface area of the polarized electrodes, however, vary depending on the diameter of the minute active carbon particles, and greatly affect the energy density and internal resistance of the electric double layer capacitor. For example, when minute active carbon particles are sintered under the same conditions, the volume resistance (specific resistance) of the resultant polarized electrodes is greater as the diameter of the minute active carbon particles is larger, and smaller as the diameter of the minute active carbon particles is smaller. Therefore, if minute active carbon particles of smaller diameter are employed, the resultant electric double layer capacitor has a smaller internal resistance. It has been found however that the minute active carbon particles of smaller diameter are fused together, resulting in a reduced specific surface area and a lower energy density.